Many devices, such as computers and televisions, require the use of a display screen. Some of these devices, such as laptop computers, require a lightweight, portable screen as a display screen. Currently, such screens typically use electroluminescent or liquid crystal display technology. A promising technology to replace these types of screens is the field emission display (FED).
Field emission displays typically include a generally planar substrate having an array of projecting emitters. In many cases, the emitters are conical projections integral to the substrate. Typically, the emitters are grouped into emitter sets where the bases of the emitters in each set are commonly connected.
A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30-120 V. The emitter sets are then selectively activated by providing a current path from the bases to the ground. Providing a current path to ground allows electrons to flow from the emitters in response to the extraction grid voltage. If the voltage differential between the emitters and extraction grid is sufficiently high, the resulting electric field extracts electrons from the emitters.
Field emission displays also include display screens mounted adjacent the substrates. The display screens are formed from glass plates coated with a transparent conductive material to form an anode biased to about 1-2 kV. A cathodoluminescent layer covers the exposed surface of the anode. The emitted electrons are attracted by the anode and strike the cathodoluminescent layer, causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the anode and the glass plate where it is visible to a viewer.
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer, which in turn depends upon the magnitude of current flow to the emitters. The brightness of each area can thus be controlled by controlling the current flow to the respective emitter set. By selectively controlling the current flow to the emitter sets, the light from each area of the display can be controlled and an image can be produced. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel." For a general overview of FED technology, see D. A. Cathey, Jr., "Field Emission Displays," Information Display Vol. 11, No. 10. pp, 16-60, October 1995, incorporated herein by reference in its entirety).
The manufacture of a FED presents several technical challenges. For example, application of phosphor to a conductive surface may involve the use of photoresist masks, as described in, for example U.S. Pat. No. 4,891,110 to Libman, et al., which patent is incorporated herein in its entirety by reference. The use of this photoresist mask may cause some problems. As described in Libman et al., the photoresist is fixed in certain areas over a conductive surface, the unfixed photoresist is then removed by a wash (using, for example, water) and the exposed conductive surface is subjected to a cataphoretic bath to apply a phosphor to the conductive surface. After that application, the fixed photoresist material must be removed, which is accomplished in the field emission display art by way of washing with, for example, a hydrogen peroxide solution. Such washing involves mechanical agitation, which can dislodge particles of phosphor, resulting in unacceptable displays. This quality problem becomes even more critical as phosphor spot size or line width shrinks to achieve higher resolutions products.
To provide contrast in ambient light, a dark matrix material may be placed in the interstitial regions between the phosphor pixels. Unfortunately, many potential matrix materials have significant disadvantages when utilized under conditions associated with the manufacture of an FED. For example, di-aqueous graphite (DAG) tends to burn when heated in the presence of oxygen. Furthermore, DAG is conveniently used only in a bake-on/lift-off process, which is not feasible for use in the manufacture of high resolution or small area FEDs. Manganese carbonate, which is light in color upon initial deposit onto a display, tends to turn brown when subjected to elevated temperature under vacuum conditions (see Libman et al.). Such browning of manganese carbonate adversely effects contrast of the FED.
Accordingly, there is a need for a method and system for manufacture of field emission displays that will not mechanically agitate the phosphor during removal of photoresist material. Furthermore, there is a need in the art for a matrix material which remains black after being subjected to conditions associated with FED manufacture, particularly at elevated temperatures under vacuum. There is also a need for a black matrix material which may be applied by deposit techniques suitable for FED manufacture. The present invention fulfills these needs and provides further related advantages.